Hollow cathode,nonthermionic electron beam source with replaceable liner



gwa mi CROSS REFERENCE SEARQH ROQ'M Dec. 23. 1969 H. STAUFFER 3,486,064

v HOLLOW CATHODE, NONTHERMIONIC ELECTRON BEAM SOURCE WITH REPLACEABLE LINER Filed March 20, 1968 2 Sheets-Sheet 1 ig 646 m l Invent 0)".- Lynn H Stauffer;

/"7/'s Attorney Dec. 23. 1969 L. H. STAUFFER HOLLOW CATHODE, NONTHERMIONIC ELECTRON BEAM SOURCE WITH REPLACEABLE LINER 2 Sheets-Sheet 2 Filed March 20, 1968 [r7 ventor'r 1. ynn H Stauffez: by ZZZ 5 14/ HA? A t t arne y.

nited States US. Cl. 313231 9 Claims ABSTRACT OF THE DISCLOSURE A hollow electrically conductive, shielded cathode structure is constructed of at least two member mechanically fastened together in a nonpermanent connection wherein at least one of the members is aneasily and quickly replaceable element which may be renewed upon the erosion thereof due to operation of the cathode. One particular embodiment of the cathode structure is cylindrical having separate side walls, a first aperture end wall and a second top plate end wall. A liner side wall of electrically conductive material having high secondary electron emission characteristics obtains a prolonged cathode lifetime; and improved cathode operating characteristics due to the increased secondary electron emission. A replaceable screw fabricated of a material such as molybdenum or tungsten in the top end wall prevents erosion of the top end wall due to ion sputtering.

My invention relates to improvements in electron beam irradiation apparatus of the gaseous or plasma type wherein the beam is generated nonthermionically, and in particular, toimprovements in the elect on beam gun assembly of the apparatus described in US. Patent No. 3,320,475 entitled Nonthermionic Electron Beam Apparatus, inventor Kenneth L. Boring, and assigned to the assignee of this application. This is a continuationin-part application of SN. 466,012, filed June 22, 1965, now abandoned, having the same title and assignee as the parent application.

The above-identified patent describes a nonthermionically emitting electron beam gun assembly which is comprised of a hollow cathode structure having imperforate side walls and a single exit aperture in an end wall through which a beamof electrons may be emitted, and an electrically conductive shield substantially surrounding the cathode. The electron gun assembly is positioned within a housing and the interior thereof is subjected to a low pressure relatively inert ionizable gas. A high negative cathode-to-hou'sing potential is applied, and interaction of the gaseous medium and negative potential produces a body of plasma within the cathode cavity. The electron beam which issues from the plasma and passes through the cathode aperture may be used in welding, brazing, melting, and annealing of material such as aluminum, copper, steel and refractory materials such as niobium and molybdenum due to the high power densities that may be obtained in the electron beam. The electron gun assembly (hereinafter also described as electron beam source) described in the Boring patent has several advantages over known thermionically emitting electron beam apparatus, notable, (1) relative insensitivity to contamination due to the subject electron gun operating in a rough vacuum as opposed to the high vacuum of thermionically emitting apparatus, (2) ruggedness and simplicity of the subject electron gun which includes the need for only a very simple electron beam focussing lens for concentrating the electron beam on a very small area of a workpiece being processed thereby, and in some atent cases may not even require such focussing lens, (3) ability to operate in a' partial vacuum (pressures to microns in hydrogen or helium) and (4) grid control of beam intensity with high speed of response.

It has been found tht the hollow cathodes employed in the Boring patent which are usually made of stainless steel or molybdenum parts permanently assembled as by welding or brazing, have a limited operating life of approximately 50 hours at high power levels (10 to 20 kilowatts input power) and at least 1000 hours at low power levels (1 to 2 kilowatts) for a 1 inch diameter cathode. It is believed that this limited life is caused by formation of a black magnetic scale, presumably iron, in the case of the sfitifile ss s teel cathode on the inner surface thereof which flakes oil and interferes with cathode operation at the high voltages in the region of 30 to 50 kilovolts. Both stainless steel and molybdenum cathodes are subject to erosion of the cathode aperture end wall (exit aperture end plate) and also on the inner surface of the nonape'rt ure end of the cathode. The erosion appears to be a result of electrostatic focussing of back-streaming ions caused by the exit aperture. This erosion by ion sputtering considerably shortens the oathode life. While the hereinbefore described hollow cathode is satisfactory when embodied in the electron gun assembly for welding, heating and irradiation applications, certain improvements may be desirable in the cathode structure to prolong the life thereof and thus not require as frequent replacement of such entire cathode.

Therefore, one of the principal objects of my invention is to provide an improved apparatus for efiiciently generatin an electron beam by nonthermionic means.

Another object of my invention is to provide an improved electron gun assembly for the electron beam apparatus.

A further object of my invention is to provide a composite cathode in the electron gun assembly for obtaining prolonged life thereof.

A still further objectof my invention is to provide such cathode for obtaining improved operation of the electron beam apparatus.

Briefly stated and in accordance with my invention, 1 p ovide a hollow, electrically conductive composite cathode structure substantially surrounded by an electrically conductive shield to forfn a unitary electron beam gun assembly which is used as an electron beam source in an electron beam irradiation apparatus. In one embodiment, the cathode structure has imperforate side walls and a single aperture" in an end wall thereof through which an electron beam may issue. The shield is concentric with the cathode and electrically insulated therefrom and is normally maintained at ground potential. The electron gun assembly is supported within a housing adapted to contain a low pressure ionizable gaseous medium. The electron gun assembly, when operable within a particular range of gas pressure and negative cathode-to-housing potential, produces a well focussed electron beam emitted f om a body of plasma generated in the cathode cavit by an interaction of the gas and electric potential. My im rovement consists of constructing the cathode structure of a side wall and at least one separate end wall member to form the ca hode cavity, one or more of such members being renewable, that is, adapted to be easily and quickly removable and replaced by a new member after e osion thereof due probably to ion sputteringto thereby prolong the life of the electron gun assembly. The side wall ard end walls are mechanically fastene together in a nonnermanent connection. In addition, a thin liner of suitable electrically conductive material may be installed along the inner surface of the cathode side wall to improve the electron emission characteristics thereof and thus obtain improved operation of the cathode. Finally, a replaceable screw fabricated of a suitable material is provided in the nonaperture end wall to prevent erosion thereof due to ion sputtering.

The features of my invention which I desired to protect herein are pointed out with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing wherein like parts in each of the several figures are identified by the same character reference and wherein:

i FIGURE 1 is an elevation view, partly in section, illustrating a first embodiment of a nonthermionically-emitting plasma electron beam source "constructed in accordance with my invention and installed in an electron beam irradiation apparatus;

FIGURE 2 is an enlarged sectional view of a second embodiment of the electron beam source;

FIGURE 3 is an enlarged sectional view of a third embodiment of the cathode structure portion of the electron beam source;

' FIGURE 4 illustrates a fourth embodiment of the cathode structure;

,FIGURE 5 illustrates a fifth embodiment of the cathode structure; and

FIGURE 6 illustrates a sixth embodiment of the cathode structure.

'Before describing the improved plasma electron beam soprce which is the subject of my present invention, a brief summary of a theory for explaining the principle of electron beam formation and ejection from the hollow cathode will be provided, attention being directed to the Boring patent for additional explanation of the theory. It.-is well known that relatively high voltages must be applied to initiate electrical breakdown (spark-over) a'cr oss gaps which are short compared with the electron mean-free-path for ionization at the gas pressure existing in the gap. In such short gaps, multiplication of ions and electrons cannot take place because of the low probability of ionizing collisions at low gas pressures. Thus, discharges at low pressure tend to seek out long paths instead of short ones as is the case at high pressures, and this phenomenon is employed to suppress radial electron emissiop f om the hollow cathode (to the housing side walls) by *closely fitting a concentric shield about the cathode and electrically insultaed therefrom. Any discharge which occurs across the gap (between the grounded shield and cathode) traverses the long path between the outer surface of the shield and the inner surface of the cathode or near the cathode beam exit aperture. This circumstance co fines positive ion bombardment largely to the inner cathode surface and to the outer surface of the cathode (exit) aperture end wall. Proper spacing of the end of the shield with respect to the plane of the cathode apertureiend wall focusses most of the ions on the cathode aperture thereby generating secondary electrons within the cathode cavity. As a result of these phenomena an ionized body of plasma forms within the cathode cavity. The interior of a cathode 7 cavity in my FIGURE 1 thus comprises a glowing body 11 of plasma or ionized gas being generated by an interaction of a low pressure gaseous medium and a sufliciently high negative cathodeto-housing electric potential. The gas pressure is in the range of 1.0 to approximately 200 microns, depending upon the particular gas employed, and the cathode-tohousing potential is generally in the range up to approximately 30 kilovolts, although for some applications it may be as high as 200 kilovolts. The body of plasma is separated from the cathode walls by a less luminous sheath which is bounded by the cathode walls. A region of high voltage gradient (cathode dark space) surrounds the aperture end 12 of the cathode externally thereof. The combi esi e fec o h P e t a distt z sn i s de the sa h d and caihode dark space allows the emergence of a stream of electrons from the plasma and initiation of electron beam formation within a particular range of gas pressure and catho-de-to-housing potential. The shield 9 being, in gereral, at the ground potential of the housing further aids in creating a suitable electric field distribution for beam mode operation, it being difficult to operate the cathode in such mode without the shield. Highly conducting gas outside the boundary of the cathode dark space acts as a virtual anode and the electrons in the beam gain most of their energy as they are accelerated in the space between cathode and dark space boundary. As the gas pressure is increased, the dark space shrinks and the electron beam current increases due to a corresponding increase of positive ion influx. Increasing the cathode voltage produces a nonlinear increase in beam current. Thus, variation of the gas pressure or cathode voltage controls the electron beam intensity.

For a desired level of beam intensity, the gas pressure and cathode voltage must each be within a critical range to obtain and maintain beam mode operation. The critical range within which beam mode operation exists is dependent primarily on the gaseous medium employed and to' a lesser degree on the cathode voltage and geometry. A typical critical pressure range for beam mode operation in an argon medium and a cathode voltage of 20 kilovolts is 5 to 10 microns for a cylindrical cathode structure 3 inches long, 1%" outside diameter, 1.5" inside diameter, aperture wall thickness of 0.125 and aperture diameter of The upper limit of beam current for each cathode is determined by the current magnitude which causes excessive heating of the aperture end wall 12 by ion bombardment. This ion bombardment or ion sputtering causes erosion of the inner surface of the cathode nonaperture end wall 13 as well as the outer surface of the aperture end wall 12. In the cathode of the Boring patent wherein the cathode is permanently assembled as a unitary structure by welding or brazing the end walls to the side walls, and thus having no conveniently renewable parts, holes as large as deep are sometimes found at the center of the nonaperture end wall after long usage. The erosion is believed to result from electrostatic focussing of the backstreaming ions due to the electrostatic lens effect of the cathode aperture. The above-described cathode having nonren ewable parts, although very useful in performing the various electron beam irradiation processes hereinabove mentioned, have been found to have limited life due to the erosion problem and the purpose of my present invention is to overcome this difficulty and thereby obtain a prolonged lifetime of the cathode as well as improved cathode operation characteristics.

Referring now to'FIGURE 1, there is shown a first embodiment of a nonthermionically-emitting plasma electron beam source having renewable parts and constructed in accordance with my invention and installed in an electron beam irradiation apparatus. The electron beam source (electron gun assembly) includes a hollow composite cathode structure illustrated as a whole by numeral 7 preferably in the form of a cylinder although other shapes such as parallelepiped and spherical may be employed, having imperforate side-l walls 22 and a single aperture 8 in the center of the bottom end wall 12 thereof through which the electron beam is emitted nonthermionically. Exit aperture 8 may be circular, or any other shape, being determined primarily by the cathode configuration and desired cross sectional shape of the emitted electron beam. The designation of portions of the cathode as bottom and top end walls and side walls asherein employed is for illustrative purposes only, it being obvious that for some applications, the cathode orientation may be different from that shown. As illustrated in FIGURE 1, cathode 7 is conveniently assembled by attaching a separate bottom (aperture) end w l 12 t9 de W 2; n a y sen s en a ap aa nt mechanical fastening manner such as, for exemplary purposes, a breechblock connection wherein end wall 12 is rotated through part of a revolution and thereupon is automatically locked in place within the protruding bottom portion of side wall 22. In like manner, top end wall 13 may also be a separate member but more permanently connected to side wall 22 by being welded or brazed thereto. However, it is often preferable to fabricate side wall 22 and top end wall 13 from a single piece of material and thereby avoid the possible problem of a defective joint. The hollow cathode comprising the side, bottom and top end walls can be constructed from an electrically conductive material that has a relatively high melting point in the order of 1500 to 3000JC. and higher, depending upon the particular beam irradiation process, to avoid melting 'at the temperature to which the cathode may be subjected at high beam intensity even though no heat source as such is utilized, and preferably must not emit significant amounts of gas at this temperature. The cathode material should also have the following characteristics (1) be resistant to spark-over between cathode and shield, (2) be resistant to erosion due to ion sputtering and (3) have high secondary electron emission. Secondary electron emission (ratio) is the number of electrons emitted from a surface per ion impact. High secondary electron emission is exhibited by low and high atomic number metallic elements whereas elements of intermediate atomic number (between 30 and 60) and carbon have low secondary emission characteristics.

Ion sputtering (ratio) is the number of target atoms removed per ion impact, and should be low for the cathode parts. Elements having relatively low sputtering ratios are carbon, molybdenum, tantalum, titanium and tungsten for example. As examples, the cathode side and top end wall fabricated from a single piece of material, as by boring out a solid rod, may be constructed from stainless steel, and for especially high temperature applications may be constructed from refractory materials such as molybdenum, tantalum or tungsten. In the case of separate side and top'end walls, side wall 22 is preferably constructed from a sheet of molybdenum for its high secondary electron emission characteristics, and the top end wall is constructed from stainless steel, although both members can be constructed from the same stainless steel, molybdenum or other refractory materials. Separate bottom end wall member 12 may also be constructed from a refractory material or stainless steel. Finally, any or all of the walls of the cathode may also be constructed from a hard dense electrographite carbon which is much less susceptible to erosion from ion sputtering than most metals, however, this carbon material is preferably employed in the top and bottom end walls which are erosion susceptible. A carbon cathode is not defined in the Boring patent since carbon parts cannot be welded or brazed. An all-carbon cathode structure is operable, especially in a helium atmosphere, but the gas pressure required is 1.2 to 1.3 times as high as required for all-stainless steel or molybdenum cathodes thereby indicating the lower secondary electron yield or carbon per incident positive ion. The cathode structure illustrated in FIGURE 1 thus includes an easily and quickly replaceable bottom end wall 12 (i.e. it is easily removable and renewable) whether it be made of a refractory material, stainless steel or carbon. The replaceable bottom end wall element permits a prolonged life for the cathode since such element upon erosion is replaced by a new one.

An electrically conductive shield 9, having perforate or imperforate side walls and an open bottom end, surrounds cathode 7 in closely spaced apart concentric relationship and is elecrically insulated therefrom. Shield 9 has the same configuration as the cathode, and thus in one embodiment is preferably cylindrical. Shield 9 may be made of sheet metal such as stainless steel and the spacing between cathode and shield is maintained sufficiently small to prevent a glow discharge in that space. The cathode-to-shield spacing is dependent; on the cathode voltage, gas pressure and cathode-shield geometry. For the 3" long cathode hereinabove described, a shield having a 2%" outside diameter and 2 /8" inside diameter has proven satisfactory. The structure of shield 9 preferably includes a means for obtaining axial adjustment of shield 9 with respect to cathode 7, a conventional clamping means 10 being illustrated in FIGURE 1. A preferable adjustment of shield 9 is a position whereby the bottom end thereof is slightly above the aperture end of the cathode, approximately 91 being satisfactory. However, for some applications it is desirable for the bottom end of shield 9 to extend below the cathode aperture end.

The electron gun assembly comprising ca hode 7 and shield-9 is positioned within a housing designated as a whole by numeral 1, preferably of cylindrical shape, although other forms may be employed. Housing 1 comprises a top end plate 2, hollow cylindrical wall 3 and bottom end plate 4 joined by well known methods. End plates 2 and 4 are constructed of an electrically conductive material such as metal, and wall 3 inay also be constructed of such material, or alternatively, may be made entirely or partially of a nonporous; transparent, heat resistant material to permit visual observation of the generated electron beam and its elfect on an irradiated material or work piece being processed by the beam. Work piece 5 is maintained in alignment with the electron beam by means of a movable support member 6, disposed on bottom end plate 4 and constructed of copper or other suitable good electrically conductive and heat conductive material. The anode of the electron beam apparatus primarily includes housing 1, shield 9 and support member 6. Suitable means (not shown) are provided to remove member 6 from housing 1 and thereby facilitate the insertion and withdrawal of material 5 being processed therein. An external beam fqrcussing lens (not shown) of electrostatic or electromagnetic type may be positioned approximately midway between the bottom of cathode 7 and work piece 5 and aligned therewith, if desired, to obtain an additional degree of electron beam focus control. 1

Cathode 7 is supported within housing 1 by a cathode stem 16 which is electrically insulated from top end plate 2 by means of high voltage insulating; bushing 17. Stem 16 is an electrically conductive solid or? hollow rigid member which may be cylindrical in form imd made of stainless steel. The solid form is employed when no internal passage is required for a cooling"? medium to remove heat being generated within cathode structure 7 by the plasma 11 within the cathode cavi'y. The cathode is connected to the stern by suitable means such as welding or brazing in the case of,permanent connections or a screw arrangement. Shield 9 'is connected to the top end plate 2 by suitable means such as a metallic tubular member 49 whereby shield is operable at the same potential (normally ground potential) as top end plate 2 relative to the cathode. The rigid cathode stem 16 maintains cathode 7 and shield 9 in the desired concentric and spaced apart relationship.

The output of a high voltage direct current power supply (not shown) providinga controlled output voltage is connected to terminals 14 and 15, housing 1 being maintained at ground potential, as illustrated, for many applications. The negative terminal 14 of the power supply is connected to cathode stem 16 whereby the cathode is operable at a relatively high negative potential with respect to the anode. The power supply output voltage as hereinabove described, is adjustable from 0 to approximately 30 kilovolts, and for some applicaions may be as high as 200 kilovolts. The power rate of the supply is dependent upon the particular application and may be in the order of 15 kilowatts for the 1 /2" diameter cathode and 50 kilowatts for a 3" diameter cathode for applications such as welding, brazing, melting and annealing of materials such as steel, aluminum, copper and refractory metals such as niobium, tungsten, tantalum and molybdenum.

A suitable relatively inert and ionizable gas such as argon, helium, nitrogen or hydrogen is introduced into the interior of housing 1 through passage 18 which may pass through any wall of housing 1, and for illustrative purposes is shown as passing through top end plate 2. Passage 18 is connected to 'a gas supply (not shown) through throttle valve 19 which regulates the rate of gas flow into housing 1. A second passage 20 is preferably located in a wall of housing 1 remote from passage 18 and is illustrated as passing through bottom end plate 4. An exhaust pumping device (not shown) is connected to passage 20 through regulating valve 21 and aids in maintaining a desired gas pressure within housing 1. Thus, possible contamination of the cathode by undesired gases generated by the irradiated material is largely prevented with such an exhaust system.

The particular improvement in the plasma electron beam source in accordance with my present invention hereinabove described having the cathode (bottom) aperture end wall 12 being a replaceable element may also be extender to the (top) nonaperture end wall 13 as illustrated in FIGURE 2. In FIGURE 2, an enlarged detail view of a second embodiment of my electron beam source is illustrated with cooling means for the cathode also being shown. In particular, cathode 7 is illustrated as having an easily and quickly replaceable nonaperture end wall 13, side wall 22 and bottom end wall 12 being shown as fabricated from a single piece of material although separate members permanently connected may also be used. The electron gun assembly is assembled as a unitary structure containing the cathode 7, shield 9, high voltage insulating bushing 17, cathode stem 16 which includes a coaxial cathode cooling passage, and a flange 23 which is attached to top end plate 2 of the electron beam irradiation apparatus by any well known gas-tight sealing technique such as the illustrated O-ring gasket and a bolt arrangement which also maintains alignment of the electron beam source in top plate 2 and assures electrical continuity between plate 2 and flange 23. The overall electron gun assembly of FIGURE 2 is preferred over the assembly of FIGURE 1 in that it is readily mountable and demountable with respect to top end plate 2 as opposed to the unitary assembly of the electron beam source and top end plate 2 in FIGURE 1. In the particular cathode structure of FIGURE 2, nonaperture end wall 13 is a solid disk and is supported on a shoulder portion of side wall 22 and held in place by a cathode cooling chamber 24 which rests thereon and forms the base of the cathode structure. Imperforate cathode side wall 22 is attached to base 24 by one or more flat'head screws 25 and thus is readily demountable therefrom. Perforate or imperforate shield 9 is concentrically positioned about cathode 7 and maintained in spaced apart relationship thereto. Shield 9 is of the same shape as cathode side wall 22 and thus is preferably cylindrical, and may be made of sheet metal such as stainless steel. As heretofore described, the spacing between cathode and shield is maintained sufliciently small to prevent a glow discharge in that space and the open bottom end of shield 9 is slightly above the aperture end of the cathode. Shield 9 is attached to flange member 23 by means of one or more set screws 26 or any other convenient readily removable connection device. The electrostatic design of cathode stem 16 is improved by extending base 24 of the cathode inside high voltage insulating bushing 17 to suppress long path electrical discharges. The extensions 27 and 28 along the outer and inner edges of cathode 7 and flange 23, respectively, also provide a short gap as compared with the electron mean-free-path to prevent electrical discharge thereacross. Hollow cathode stem 16 is provided with a coaxial cooling passage therein, a cooling medium such as water being indicated as flowing down through the inner fluid passage 29 and flowing up through the outer passage 30. The cathode stem 16 which forms the outer portion of the exit fluid passage 30 for the cooling medium is suitably joined to the upper portion of cathode base 24 and the inner part of the coaxial cooling passage extends further into a hollow portion 32 of cathode base 24 for maximum transfer of heat fron i the cathode to the cooling medium flowing therein. A suitable fluid-tight enclosure 33 is provided at the top end of cathode stem 16 to provide an exit passage 34 for the cathode cooling medium. A suitable brazed flexible ceramic-to-metal seal 37 is used to connect insulator 17 to flange 23. Although insulator 17 may be supported in direct contact with flange 23, the flexible connection 37 is preferred to eliminate any possible damage to the insulator due to unequal temperature expansions of the two components. A metal cap 38 is joined to the top of insulator 17 by a suitable process such as brazing, and a collar member 39 is then brazed to cathode stem 16 and soldered to cap 38 to provide a support for maintaining stem 16 concentric with insulator 17 and thereby maintain equal spacing around the periphery between cathode 7 and shield 9.

In the FIGURE 2 embodiment, cathode side wall 22, aperture end wall 8 and nonaperture end wall 13 may be made of the same materials as specified for side wall 22 in FIGURE 2. Cathode base 24 is made of an electrically conductive, good heat conductive material such as copper to permit rapid transfer of heat from the cathode top and side walls to the cooling medium.

The composite cathode structures of FIGURES 1 and 2 have easily and quickly replaceable aperture end wall 12 and nonaperture end wall 13, respectively, the particular electron beam gun assembly illustrated in FIGURE 2 being of preferred and more practical design than the simplified assembly of FIGURE 1. The replaceable end walls 12 and 13 of FIGURES 1 and 2, respectively, may each be constructed of materials such as molybdenum, or, for lower temperature applications, stainless steel, or, for maximum erosion resistance a material such as carbon. In either case, the cathode end wall member being renewable obtains a longer cathode life. The use of easily replaceable nonaperture and {aperture cathode end wall portions may be combined as illustrated in FIGURE 3 wherein both cathode end walls are easily removable and renewable to obtain an even greater prolonged life of the cathode structure. Since the two cathode end walls are the members primarily susceptible to erosion by ion sputtering, they are preferably fabricated from a low erosion material such as carbon (as illustrated), although they may also be fabricated from molybdenum or even stainless steel, if desired. Aperture end wall 12 is illustrated as being threaded into a shoulder portion of side wall 22 in FIGURE 3 for purposes of indicating another nonpermanent connection means. Cathode base 24 which is illustrated in part, is preferably made of copper as heretofore described. I ll FIGURE 4 illustrates a fourth embodiment of a cathode structure having renewable parts. In the case of the FIGURE 4 cathode, side wall 22 and aperture end wall 12 are illustrated as being fabricated from a single piece of carbon although the other side wall materials previously described may also be utilized. Nonaperture end wall 13 constructed from molybdenum although it may also be constructed from carbon or even stainless steel, if desired. Since the inside surface of side wall 22 is the primarysource of secondary electron emission, and carbon is poor secondary electron emitter, a thin liner (sheet) of molybdenum 36 is installed adjacent the inner surface 9f the carbon side walls. The spacing between the inner side wall surface and liner 36 is up to ap proximately .01 inch since upon operation of the cathode the heat expands the liner such that it comes into electrical contact with the cathode side wall at many points. The molybdenum liner may be of thickness in the order of .005 inch. The cathode structure of FIGURE 4 is well suited for lower voltage operation below 30 kilovolts wherein (cathode-to-shield) spark-over is minimized and thus the carbon side wall 22 is compatible. It is to be noted that the cathode structure of FIGURE 4 has two replaceable elements, top wall 13 and liner 36.

A fifth embodiment of my composite cathode structure is illustrated in FIGURE 5 and represents a structure having the maximum life and best operating characteristics of all the cathodes herein described. Side wall 22 is prefeably constructed of stainless steel since the outer surface thereof is subjected to high electric field strength (cathode-shield) and stainless steel has a higher resistance to spark-over than does carbon; Aperture and nonaperture end walls 12 and 13, respectively, are each preferably constructed of carbon since it has a higher resistance to erosion from ion sputtering than does molybdenum or stainless steel. A molybdenum liner 36 is fitted adjacent the inner side of cathode side wall to insure high secondary electron emission from the inner side of the cathode. In operation of the cathode, the molybdenum of liner 36 sputters and soon coats most of the inner surfaces of end walls 12 and 13 which improves the electron emission characteristics of these carbon sufaces. The cathode structure of FIGURE 5 is especially well adapted for high voltage operation "above 30 kilovolts wherein spark-over between the cathode and shield may be encountered, since stainless steel has an exceptionally high resistance to such spark-over. It should be noted that a molybdenum liner may also be used in conjunction with a molybdenum cathode side wall for purposes of increasing the secondary electron emission characteristics. The liner, being closer to the hot plasma within the cathode cavity and being thermally isolated from the cathode side wall except for radiation losses (but electrically connected thereto at many points), attains a higher temperature than the side wall, thereby increasing the secondary electron emission from the liner. Finally, a screw 50 fabricated from a low sputtering material having a high melting point temperature (such as molybdenum, tungsten or carbon) is provided through the center of nonaperture end wall 13 and into cathode base 24 to stop the axially concentrated back-streaming ions which are primarily responsible for erosion of the end wall 13 by ion sputtering. Screw 50 has a head diameter dimension in the order of% to /2 the cathode inner diameter, and in some cases can even be substantially equal to the cathode inner diameter and in such latter event end wall 13 is not needed. In cases wherein the nonaperture end wall 13 is not supported on shoulder portions of the cathode side wall, screw 50' also provides the means for securing end wall 13 in place against cathode base 24. Screw 50 develops a cavity (erodes) due to the back-streaming ion beam after approximately 50 to 100 hours of cathode operation and can then be easily'and quickly replaced by unscrewing thereof. In the event that cathode side wall 22 is fabricated of carbon, a liner of a material such as molybdenum can also be used.

FIGURE 6 illustrates a sixth embodiment of my improved cathode structure. The nonaperture end wall 13 and side wall 22 are fabricated from one piece of material such as molybdenum or any of the materials hereinabove mentioned. End wall 13 is much thicker in this embodiment than in the first five embodiments, thereby providing a longer cathode life. Aperture end wall 12 may be fabricated of carbon, molybdenum or any other materials previously described, and fitted into place by any of the nonpermanent connection means hereinabove described. In particular, if wall 12 is adapted to securely fit in place by means of a shoulder portion as in the case of FIGURE 5, then two opposite edges of such shoulder must be slightly flattened for ease of insertion into the cathode. The top (nonaperutre) end of the cathode is threaded 51 to permit the entire cathode to be easily demounted. An 0 ring 52 of rubber or other suitable material prevents the cooling medium from leaking from the cooling tube 32 into thecathode chamber. The maximum erosion of end wall 13 due to ion sputtering is generally limited to the very center therof. Since screw 50 is relatively expensive, it would be preferred to avoid this extra expense. For this purpose, a replaceable pin or rod 53 may beutilized as the core of screw 50. Pin 53 has a small diameter which overlaps the most erosion-susceptible region of end wall 13, and as one example is Ms" diameter for a cathode having an inner diameter of 1.0". The upper end 54 of pin 53 is enlarged and fitted into a recess of the shank-end of screw 50 such that pin 53 is replaced by unscrewing screw 50 and removing pin 53 therefrom. Pin 53 is fabricated from a low sputtering mateiral such as carbon or molybdenlim, obviously also having a highimelting point temperature. The length of pin 53 protruding into the cathode chamber is not critical, the longer the length thereof the" longer its life before complete erosion thereof due to ion sputtering.

From the foregoing description, it can be appreciated that my invention attains the objectives set forth and makes available an improved electron beam source for generating electron beams by nonthermionic means. The improved electron beam source is anelectron beam gun assembly which includes a single aperture hollow composite cathode structure that is operable at a relatively high voltage in a low pressure gaseous medium, and a shield substantially surrounding the cathode and operable in general at ground potential, the improvement comprising the assembly of the cathode with one or more renewable parts such that upon erosion of any such parts by ion sputtering or for any other reason, such eroded part may be easily and quickly repalced by a new one and the cathode life thereby substantially prolonged. The electron beam source is useful in many electron beam irradiation applications and especially for processing materials in controlled environments wherein the controlled environment may be the gaseous medium within which the cathode is operable. My invention also provides a more economical cathode structure, especially in embodiments employing relatively expensive molybdenum as the cathode side wall member and (1) as replaceable molybdenum liner adjacent thereto, this arrangement permitting continued use of the side wall member and periodic replacement of the liner which provides improved electron emission characteristics, or (2) replaceable carbon end wall members, this arrangement permitting periodic replacement of the end wall members which are especially subject to erosion by ion sputtering or (3) a combination of a replaceable molybdenum side wall liner and carbon end wall members.

Having described six specific embodiments of my improved electron gun assembly, it is believed obvious that modification and variation of my invent-ion is possible in the light of the above teachings. Thus, various configurations of the cathode and shield may be employed, the configurations of these two structures in each particular application preferably being the same. Also, more than one beam exit aperture may be utilized. Further, the particular materials from which the various parts of the cathode structure are fabricated are not limited to the molybdenurn, stainless steel, carbon and other materials herein described but may include virtually any material that has a relatively high melting point and does not emit significant amounts of gas at the temperature to which the cathode is subjected during operation thereof. The outer surface of the cathode side walls should especially the resistant to spark-over between cathode and shield and the inner surface have high secondary electron emission characteristics. The cathode end walls should especially be resistant to erosion due to ion sputtering. The particular material used for each portion of the 11 cathode may be a single material as hereinbove described or any combination of suitable materials in a mixture or separate laminated form having the above-described desired characteristics. The particular structure of insulator 17 and cathode stem 16 for supporting the electron gun assembly within housing 1 may have many forms since the particular description and illustration in FIGURES 1 and 2 is not deemed to be a limitation thereof. Finally, it is to be understood that the housing itself, or a recess therein, as described in. the Boring patent, may provide the function of shield 9, in which case the shield as such is omitted and a part of the housing is then the shield. It is, therefore, to be understood that changes may be made in the particular embodiments as described which are within the full intended scope of the invention as defined by the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron beam source for use in an electron beam irradiation apparatus comprising a hollow cathode structure having side wall and first and second end wall portions defining a cathode cavity, said side wall and first end wall portions being of imperforate construction, said secondend wall portion having an aperture through which an electron beam may issue, said side and end wall portions constructed from electrically conductive material having a relatively high melting point temperature above 0O C. and further characterized as emitting amounts of gas at the temperature to which the cathode may be subjected at high beam intensity, at least one of said cathode wall portions being a separate member fastened to an adjacent cathode wall portion in a nonpermanent manner and being easily and quickly replaceable to thereby obtain prolonged cathode life upon erosion thereof, said cathode structure further comprising a replaceable liner of electrically conductive material having a relatively high melting point temperature in the range of 1500 to 3000 C. and high secondary electron emission characteristics such as those exhibited by the metallic elements of atomic number less than 30 or greater than 60 and installed adjacent the inner side of the cathode side wall portion to obtain prolonged life thereof and also improve cathode operating characteristics due to the increased secondary electron emission, and an electrically conductive shield substantially surrounding said cathode structure in spaced apart and concentric relationship.

2. The electron beam source set forth in claim 1 wherein said cathode first end wall portion is a separate easily replaceable member.

3. The electron beam source set forth in claim 1 wherein said cathode second end wall portion is a separate easily replaceable member.

4. The electron beam source set forth in claim 1 wherein said cathode first and second end wall portions are separate easily replaceable members.

5. The electron beam source set forth in claim 2 wherein the nonpermanent connection of said first end wall portion to said side Wall portion is a shoulder portion of said side wall portion providing support for said first end Wall portion.

6. The electron beam source set forth in claim 1 wherein said liner is fabricated of molybdenum and has a thickness in the range of .005 to .010 inch.

7. The electron beam source set forth in claim 4 wherein said first and second end Wall portions are fabricated of carbon to thereby obtain prolonged cathode life due to the higher resistance of carbon to erosion from ion sputtering.

8. The electron beam source set forth in claim 1 and further comprising a screw fabricated of a low sputtering material having a relatively high melting point temperature in the range 1500 to 3000 C.- provided through the center of said first end wall portion to stop axially concentrated back-streamingions responsible for erosion of said first end wall portion by ion sputtering, said screw being easily and quickly replaced upon erosion thereof thereby avoiding erosion of said first end wall portion.

9. The electron beam source set forth in claim 8 wherein the head of said screw is within the cathode cavity defined by said side wall, first and second end wall portions, and the head diameter is in the range of /3 to /2 the cathode inner diameter.

References Cited UNITED STATES PATENTS 2,888,591 5/1959 Schmidt et a1. 3l3337 X 2,932,755 4/1960 Jeppson 3l385 2,964,678 12/1960 Reid 313237 X 3,210,518 10/1965 Morley et al 219-12l 3,320,475 5/1967 Boring 313-231 X JAMES W. LAWRENCE, Primary Examiner PALMER C. DEMEO, Assistant Examiner U.S. c1. X.R. 219 121; 313 237 

